1. Field of the Invention
The present invention relates to a gas sensor having a ventilation structure for holding a detecting element that detects a gas to be detected inside a metal shell or an outer cylinder, and for securing air permeability between the inside and outside of the outer cylinder.
2. Description of the Related Art
Conventionally known is a gas sensor provided with a detecting element that detects a specific gas component (for example, oxygen, etc.) in exhaust gas discharged from an internal combustion engine, using a solid electrolyte made of ceramic, such as zirconia. For example, a detecting element of an oxygen sensor that detects oxygen has a configuration in which a detecting electrode exposed to exhaust gas and a reference electrode exposed to a reference gas (typically the atmosphere) are formed on a solid electrolyte as a pair so as to sandwich the electrolyte. The detecting element detects oxygen in the exhaust gas by an electromotive force generated between the subject electrodes according to a difference in oxygen partial pressure between the exhaust gas and the reference gas (atmosphere).
When such an oxygen sensor is attached to an exhaust pipe of an internal combustion engine, the detecting element is held in a metal shell in a state where it is exposed directly or indirectly to the outside so that the detecting electrode may be exposed to exhaust gas that flows through the exhaust pipe. Also, the detecting electrode and the reference electrode are separated by an outer casing fixed to the metal shell, and the metal shell. Further, a lead wire for extracting a detection signal of the detecting element is pulled outside of the outer casing, and a plug member for closing an outlet for the lead wire is attached to the outlet.
The plug member (grommet) is provided with an atmosphere communication hole (through hole) for securing air permeability between the inside and outside of the outer casing external to the exhaust pipe so as to introduce the atmosphere toward the reference electrode, in addition to lead wire insertion holes that allow lead wires (a sensor output lead wire and a heater lead wire) to be inserted therethrough. Also, the atmosphere communication hole of the plug member is provided with a filter member for introducing the atmosphere into the outer casing, but which prevents water droplets or the like from entering the outer casing (for example, refer to JP-A-2006-208165).
3. Problems to be Solved by the Invention
However, in the oxygen sensor of JP-A-2006-208165, the filter member is directly exposed to the outside environment. Typically, since the oxygen sensor is disposed near a road surface of an automobile, the filter member is susceptible to external impact, such as contact with plants and trees, or collision with scattering stones, and the filter member may become damaged. Also, if the filter member is damaged, and if the oxygen sensor becomes wet, there is a possibility that water may enter the oxygen sensor. This in turn causes an abnormality in the output of the oxygen sensor, or short-circuiting of a terminal.